Method for treating waste by hydrothermal oxidation

ABSTRACT

A method for oxidizing organic matter contained in an aqueous effluent and an installation for implementing the method. The method comprises the following steps: injecting into a tubular body the aqueous effluent; bringing the aqueous effluent to a pressure P 1,  corresponding to the critical pressure of the aqueous effluent; bringing the aqueous effluent to a temperature T 1;  and injecting into the tubular body at n points spaced apart from one another, n fractions of at least an oxidizing composition, so that a portion of the thermal energy produced by the oxidation reaction increases the temperature of the reaction mixture from said temperature T 1  to temperature T 2 &gt;T 1  according to an increasing curve, whereby the organic matter is oxidized, the reaction mixture continuously developing from a sub-critical liquid state to the supercritical domain.

CROSS REFERENCE TO RELATED APPLICATION

This is the 35 USC 371 national stage of international applicationPCT/FR01/02782 filed on Sep. 7, 2001, which designated the United Statesof America.

FIELD OF THE INVENTION

The present invention relates to a process for the hydrothermaloxidation of waste, in particular but not exclusively organic substancespresent in aqueous effluent, and to a plant intended for theimplementation of said process.

Applications of the invention are in particular, but not exclusively,the conversion of the organic substances present in low amounts inaqueous effluents originating in particular from food processingindustries. These aqueous effluents may also comprise dissolved salts.The organic substances are converted to gases capable of beingincinerated to provide energy or to gases capable of being released intothe atmosphere without danger.

BACKGROUND OF THE INVENTION

Processes for the conversion of organic waste present in an aqueousphase are known. In particular, it is known to bring the water/organicwaste mixture to temperatures and pressures such that the water exceedsits critical point, thus resulting, where an oxidizing substance ispresent in the mixture, in the decomposition of the waste to simplechemical components of the CO₂ and H₂O type.

However, when the water/organic waste mixture, to which amounts ofoxidizing agent capable of oxidizing all the waste have been added, iscompressed and heated so that the water exceeds its critical point, theoxidation reaction which takes place produces large amounts of thermalenergy which can affect the integrity of the walls of the reactor inwhich the reaction takes place.

SUMMARY OF THE INVENTION

The same consequences with regard to the walls of the reactor areobserved when the water/organic waste mixture is compressed and heatedprior to the introduction of the oxidizing agent into the mixture.

On the other hand, when the oxidizing agent is introduced before havingbegun to compress and heat the mixture, hot spots can appear in thereactor. The latter are due essentially to the fact that the solubilityof the oxidizing agent and its heat capacity are not constant accordingto the temperature and pressure conditions of the mixture. Thus, theconcentration of oxidizing agent dissolved in the mixture is nothomogeneous in the reaction medium and the oxidation reaction produces agreater amount of thermal energy in the regions with a greaterconcentration of oxidizing agent.

In addition to the fact that the appearance of these hot spots canaffect the walls of the reactor, the poor distribution of the oxidizingagent in the reaction medium leads to a mediocre yield of the reactionfor the decomposition of the organic waste.

To overcome the localized overheating of the reactor, the idea wasconceived of injecting oxygen and water simultaneously and fractionwisealong the reactor, so that the oxygen oxidizes the organic matter andthat, simultaneously, the water lowers the temperature of the reactionmedium. However, this solution does not allow optimum decomposition ofthe organic matter as the oxidation rate is decreased by thesimultaneous lowering of the temperature. In addition, the thermalprofile of the reactor exhibits a curve which alternately increases andthen decreases at each injection, which reduces the overall yield of thereactor.

A first aim of the present invention is to provide a process for theoxidation of organic compounds present in aqueous effluents which makesit possible to overcome the abovementioned disadvantages.

This aim is achieved, in accordance with the invention, owing to thefact that the process comprises the following stages:

-   said aqueous effluent, comprising a predetermined amount of organic    substances, taken under initial pressure and temperature conditions,    is injected into a tubular body exhibiting an inlet and an outlet,-   said aqueous effluent is brought to a pressure P1 corresponding at    least to the critical pressure of said aqueous effluent, said    pressure P1 being greater than the initial pressure,-   said aqueous effluent is brought to a temperature T1 greater than    the initial temperature by heating means applied in a zone of said    tubular body,    and in that n fractions of at least one oxidizing composition, the    sum of which corresponds to the amount of oxidizing composition    necessary for the oxidation of said predetermined amount of organic    substances, are injected into said tubular body at n points spaced    apart from one another, so that a portion of the thermal energy    produced by the oxidation reaction increases the temperature of the    reaction mixture from said temperature T1 to the temperature T2>T1    according to an increasing curve between said zone of said tubular    body and the nth injection point, whereby said organic substances    are oxidized, said reaction mixture continuously evolving from a    subcritical liquid state to the supercritical region.

Thus, one characteristic of the process for the oxidation of the organicsubstances is the gradual injection of the exclusively oxidizingcomposition, via the n injection points, into the reaction mediumflowing through the tubular body. In that way, the oxidation of theorganic substances present in the aqueous effluent is carried outgradually as the reaction mixture flows through the tubular body and thethermal energy produced by the oxidation reaction at each injection ofoxidizing composition is partially dissipated between the injections,which prevents excessively intense production of energy which woulddamage the internal walls of the tubular body. And, there is no need toinject a substance capable of simultaneously cooling the reaction mediumduring the reaction.

The oxidizing composition is very clearly capable of comprising othercompounds which have no specific effect on the reaction medium.

A portion of the total thermal energy produced by the oxidation of allthe organic substances is imparted to the reaction medium, the pressureP1 of which is greater than the critical pressure of the aqueouseffluent, which allows it to gradually evolve from a subcritical statein the liquid phase up to the supercritical region, without passingthrough the gas phase. When the reaction mixture is in the supercriticalregion, the notion of phase disappears and the organic substances whichwere not oxidized between the injections of oxidizing composition areoxidized in this region.

Advantageously, the pressure P1 of said aqueous effluent is greater than23 MPa and the temperature T1 of said effluent is between 370 and 520°K. In this temperature and pressure region, the aqueous effluentcomprising the organic substances is in a subcritical liquid phase wherea portion of these substances is oxidized.

According to a specific embodiment of the invention, said portion ofthermal energy produced by the oxidation reaction increases thetemperature of said reaction mixture to a temperature T2 of less than800° K. Thus, although the temperature of the reaction mixture iscapable of being greater than 800° K. after the nth injection point,since the nth fraction of oxidizing composition reacts with theremaining organic substances, this amount of energy is insufficient todamage the internal wall of the tubular body. Thus, a substantialportion of the organic substances is oxidized before the reactionmixture reaches the temperature T2 and, as the final part which isoxidized by the final fraction of oxidizing composition is low, thetemperature of the reaction mixture will be very slightly greater thanT2. In addition, the heat capacity of water is maximum for a temperatureof between 650° K. and 700° K., which makes possible significantabsorption of the thermal energy given off by the oxidation reaction inthis temperature range through which the reaction medium passes. Thewalls of the reactor will be proportionally less affected thereby.

According to another specific embodiment of the invention, threefractions of an oxidizing composition are injected into said tubularbody at three points spaced apart from one another. The first fractionis injected when the temperature of the aqueous effluent has reached thetemperature T1, the second fraction is injected so that the aqueouseffluent reaches the temperature T2 and, when it has reached it, thethird fraction is injected.

Advantageously, the first fraction of oxidizing composition is injectedafter said aqueous effluent has reached the temperature T1, so that thereaction for the oxidation of the organic substances begins onlydownstream of said zone of said tubular body where said heating meansare applied.

According to yet another specific embodiment of the invention, saidtubular body exhibits a plurality of portions with cross sections ofdifferent sizes. This configuration makes it possible to alternatelyinsert narrower portions of tubular body, in which portions theoxidizing composition is injected, and wider portions, where theoxidation reaction takes place. Thus, the residence time of the reactionmixture in the wider portions is greater, which makes it possible toincrease the reaction time and thus the reaction yield between eachinjection of oxidizing composition.

According to an advantageous arrangement, a portion of the thermalenergy produced by said oxidation is imparted to said aqueous effluent,taken under the initial pressure and temperature conditions, in order tobring it to said temperature T1. In that way, it is not necessary toprovide additional heating means to bring said aqueous effluent from theinitial temperature to the temperature T1, which improves the totalenergy balance of the process according to the invention. Only lowstrength initiating heating means are needed.

Preferably, the oxidizing composition injected into the tubular body isoxygen, which makes it possible to convert the organic substances at anadvantageous cost. However, hydrogen peroxide can be used in certainspecific situations where the costs of supplying are advantageous wherewhen the conditions for implementing the process requires an oxidizingcomposition with a greater solubility in water.

According to a particularly advantageous arrangement, at least one ofthe fractions of oxidizing composition is composed of an oxidizingcomposition which is different in nature from the other fractions. Thus,for example, it is possible to benefit from the technological advantagesof hydrogen peroxide, in a first part of the reactor, and from the costadvantages of oxygen, in the second part.

According to a particularly advantageous embodiment, the process inaccordance with the invention additionally comprises the followingstages: said aqueous effluent and the salts present therein arerecovered at said outlet of said tubular body, the pressure of saidaqueous effluent is lowered from said pressure P1 to a pressure P0,between atmospheric pressure and said pressure P1, so as to reduce inpressure said aqueous effluent to convert all the salts to the solidstate and said aqueous effluent to the vapor state; the salts arerecovered in the solid state; and said aqueous effluent is recovered inthe vapor state, whereby said aqueous effluent and the salts presenttherein are physically separated.

A second aim of the present invention is to provide a plant intended forthe implementation of the process for the oxidation of the organicsubstances present in aqueous effluents. The plant comprises:

-   means for injecting said aqueous effluent, comprising a    predetermined amount of organic substances, taken under the initial    pressure and temperature conditions, into a tubular body exhibiting    an inlet and an outlet,-   means for bringing said aqueous effluent to a pressure P1 greater    than the initial pressure,-   heating means, applied in a zone of said tubular body, for bringing    said aqueous effluent to a temperature T1 greater than the initial    temperature, and-   means for injecting n fractions of an oxidizing composition, the sum    of which corresponds to the amount of oxidizing agent necessary for    the oxidation of said predetermined amount of organic substances,    into said tubular body, where said aqueous effluent is at least at    the pressure P1, at n points spaced apart from one another, so that    a portion of the thermal energy produced by the oxidation reaction    increases the temperature of the reaction mixture from said    temperature T1 to the temperature T2>T1 according to an increasing    curve between said zone of said tubular body and the nth injection    point, whereby said organic substances are oxidized, said reaction    mixture continuously evolving from a subcritical phase state to a    supercritical phase state.

The tubular body is advantageously composed of a tube exhibiting aninlet orifice, into which said aqueous effluent is injected, and anoutlet orifice, by which said oxidized organic substances escape. Saidtube can be straight, when the process can be carried out in a shorttubular body, but it can also be arranged helically, so as to reduce theoverall dimensions of the reactor.

Preferably, the means for injecting said aqueous effluent comprise apump capable of compressing said aqueous effluent to a pressure ofgreater than 23 MPa, said pump being connected to said inlet orifice.Thus, the pump, which also comprises an orifice for entry of the aqueouseffluent and an orifice for injection under pressure, injects saidaqueous effluent into the tubular body. The pressure of the effluent inthe tubular body is relatively constant and greater than 23 MPa, atleast in the portion where the oxidation reactions take place.

Said heating means, applied in said region of said tubular body,advantageously comprise a thermoelectric generator integral with saidtubular body. Thus, the thermoelectric generator attached to the tubularbody makes it possible to preheat the aqueous effluent which isinjected.

Said heating means, applied in said zone of said tubular body,preferably comprise a heat exchanger integral with said tubular body,the heat source of which is provided by a portion of the thermal energyproduced by said oxidation reaction. This is because the oxidationreaction produces thermal energy, at least a portion of which canincrease the temperature of the reaction medium and a portion of whichcan be recovered and used to bring the aqueous effluent to thetemperature T1.

According to a specific embodiment of the invention, the means forinjecting a fraction of oxidizing agent into said tubular body comprisea variable flow rate injector emerging in said tubular body, thepressure of oxidizing agent in said injector being greater than P1. Theinjector can be fed with oxidizing composition via a pump capable ofcompressing the oxidizing composition to a pressure of greater than P1or via a tank comprising the composition under a pressure also greaterthan P1.

According to a specific embodiment of the invention, the means forinjecting the oxidizing agent into said tubular body comprise threeinjectors, spaced apart from one another, emerging in said tubular body.

The first point for injection of the oxidizing composition isadvantageously situated between said outlet orifice of said tubular bodyand said zone of said tubular body where said heating means are applied,close to this zone.

According to a specific embodiment, the oxidation plant additionallycomprises: means for recovering said aqueous effluent and the saltspresent therein at said outlet of said tubular body, means for loweringthe pressure of said aqueous effluent from said pressure P1 to apressure P0, between atmospheric pressure and said pressure P1, so as toreduce in pressure said aqueous effluent to convert all the salts to thesolid state and said aqueous effluent to the vapor state; means forrecovering the salts in the solid state; and means for recovering saidaqueous effluent in the vapor state, whereby said aqueous effluent andthe salts present therein are physically separated.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and distinctive features of the invention will emergeon reading the description given below of specific embodiments of theinvention, given by way of indication but without implied limitation,with reference to the appended drawings, in which:

FIG. 1 is a diagrammatic view of the plant intended for theimplementation of the process according to the invention, comprising npoint for injection of the oxidizing composition,

FIG. 2 is a view of the thermal profile of the reaction media as afunction of the points for injection of oxidizing compositioncorresponding to the diagrammatic view of FIG. 1,

FIG. 3 is a diagrammatic view of the plant intended for theimplementation of the process in accordance with the invention accordingto a specific embodiment where tubular body comprises three injectionpoints, and

FIG. 4 is a view of the thermal profile of the reaction media as afunction of the injection points corresponding to the diagrammatic viewof the plant represented in FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

Reference will be made to FIG. 1 in describing the plant forimplementation of the process for the oxidation of the organicsubstances present in the aqueous effluent.

The aqueous effluent comprising the organic substances to be convertedis stored upstream of the plant for implementation of the process in atank 10. The aqueous effluents are generally composed of industrial ormunicipal sludge or of aqueous liquors resulting from industrialprocesses.

A pump 12, the entry orifice 14 of which is connected via a pipe 16 tothe lower end of the tank 10, is capable of pumping the aqueous effluentand of injecting it under pressure into a tubular body 18 at its inletorifice 20. The pump 12 is capable of injecting the aqueous effluentinto the tubular body 18 under a pressure of greater than 22 MPa, whichcorresponds substantially to the critical pressure of water.

The tubular body 18 is equipped with a thermoelectric generator 22 whichat least partially surrounds the external wall of the tubular body closeto the inlet orifice 20 into which the aqueous effluent is injected. Thethermoelectric generator 22 is composed of a heating resistor capable ofproducing enough thermal energy to raise the temperature of the aqueouseffluent which passes through the tubular body 18.

It goes without saying that any other means capable of producing thermalenergy is capable of being used, in particular means operating with gasor other fuels.

This contribution of energy to the aqueous effluent is necessary toinitiate the reaction for the oxidation of the organic substances, whichtakes place as soon as a first fraction of oxidizing composition isinjected at the injection point 24. This injection point 24 is situatedin the tubular body 18 downstream of the thermoelectric generator 22. Ina specific embodiment, the injection of the first fraction of oxidizingcomposition is carried out upstream of the heating means after the inletorifice of the tubular body, so as to dissolve a portion of oxidizingcomposition in the aqueous phase at the initial temperature.

At the injection point 24, an injector (not shown) passes through thewall of the tubular body 18 and emerges in the port of the latter. Theinjector is connected to a pump 26 or to a tank (not shown) by means ofa pipe 28. The pump 26 or the tank is capable of delivering a fractionor oxidizing composition under a pressure greater than the pressure ofthe moving effluent of the tubular body 18. This is because thiscondition is necessary for the oxidizing agent to be injected into thetubular body 18.

The oxidizing composition can be composed of any substance capable ofpulling electrons from the organic substances. The least expensiveoxidizing agent is oxygen and it is easy to inject it by means of aninjector. Other oxidizing agents can be used, such as hydrogen peroxideor such as nitric acid, which exhibits the advantage of decomposingnitrogen oxides and of producing water and nitrogen.

A second point 30 for injection of the oxidizing composition situatedclose to the first injection point 24, downstream, makes possible theinjection of a second fraction of the oxidizing composition. The meansemployed for injecting the oxidizing composition are identical to themeans employed for carrying out the injection at the first point 24.

The number of fractions of oxidizing composition to be injected into thetubular body 18 can be varied as a function of the concentration oforganic substances present in the aqueous effluent and of the amount ofoxidizing agent necessary for the oxidation of all the organicsubstances and as a function of the geometry of the tubular body. Aspecific embodiment of the invention in which the plant comprises threepoints for injection of oxidizing composition will be described in moredetail in the continuation of the description.

According to an advantageous arrangement, when the temperature of thereaction medium is increased after the injection of the first fractionof oxidizing agent, at least two types of oxidizing composition areused. Hydrogen peroxide is injected first, due to its high oxidizingpower, and then oxygen fractions are injected into the other injectionpoints. The reaction having begun, oxygen can react in an optimumfashion. According to this embodiment, the cost balance of the reactoris improved as oxygen is less expensive than hydrogen peroxide.

In accordance with FIG. 1, the plant comprises a final point 32 forinjection of the oxidizing composition, known as the nth injectionpoint.

In order for the oxidation reaction to be substantially complete, thatis to say for all the organic substances to have oxidized, it isnecessary for the amount of oxidizing agent injected into the aqueouseffluent to be at least equal to the amounts of oxidizing agentcorresponding to the stoichiometry of the reaction for the oxidation ofthe organic substances. Thus, the sum of the fractions of oxidizingcomposition injected into the tubular body 18 is at least equal to thestoichiometric amount of oxidizing agent for the reaction for theoxidation of a given amount of aqueous effluent. Very clearly, theoxidative process takes place continuously and the reasoning which isapplied for given amounts can be transposed to the continuous operationby using measurements of flow rates.

When the reaction is complete and when the organic substances compriseonly compounds based on carbon and on oxygen, the oxidation products arecomposed of carbon dioxide and of water. These oxidation products arereleased at the end of the tubular body 18 at an outlet orifice 34.

The process according to the invention makes it possible to convert toinorganic compounds an organic load present in an aqueous effluent, forexample to produce water and carbon dioxide. In this case, the reactionproducts can certainly be released to the atmosphere without damage tothe environment or can be recovered in order to be used as reactant, ifthe content of carbon dioxide is sufficient.

The products from the reaction for the oxidation of the organicsubstances can also be released to the atmosphere if, for example, theycomprise nitrogen resulting from the decomposition of nitrogen oxide bynitric acid. On the other hand, if the organic substances comprisechlorine, the hydrogen chloride originating from the reaction will haveto be recovered by chemical conversion.

As will be described in more detail in the continuation of thedescription, the tubular body is a priori at its maximum temperature inthe zone situated after the nth injection point. Thus, it is possible torecover this thermal energy by means of a first exchanger 36 situated insaid zone where the temperature is at a maximum in order to transfer itupstream of the tubular body 18 by means of a second exchanger 38. Thisthermal energy, transferred close to the inlet orifice 20 of the tubularbody 18, makes it possible to supplement or to replace thethermoelectric generator necessary for the preheating of the aqueouseffluent. This configuration is of economic advantage in that it reducesthe amount of energy necessary for the implementation of the process.

After having described the constituent components of the plant necessaryfor the implementation of the process in accordance with the inventionwith reference to FIG. 1, the process for the oxidation of the organicsubstances present in the effluent and the thermal profile of thereaction media will now be described with reference to FIG. 2. Thisfigure is situated straight below the device of FIG. 1 in order for thethermal profile of the reaction medium to correspond to the variousportions of the tubular reactor 18.

The aqueous effluent is first of all compressed by means of the pump 12before being injected under a pressure of greater than 22 MPa into theinlet orifice 20 of the tubular body 18. The compression, which makes itpossible to raise the temperature of the aqueous effluent, issupplemented by the second heat exchanger 38, if the plant is undernormal operating conditions, or by the thermoelectric generator 22, ifthe plant is in a transient state. The reaction medium, initially at thetemperature Ti, is thus brought to the temperature T1 along a slope 40in accordance with the thermal profile of FIG. 2.

The temperature T1 is between 370 and 520° K., while the pressure of thereaction medium is kept constant, which makes it possible to retain thereaction medium in the liquid phase. The reaction medium retains aconstant temperature T1 for a transient period corresponding to theplateau 42.

Subsequently, a first fraction of the oxidizing composition is injectedat the first injection point 24 and the temperature of the reactionmedium increases according to the slope 44 to reach the temperature T1₁. This is because the oxidation of the organic substances by theoxidizing composition is exothermic and, consequently, imparts energy tothe reaction medium.

A second fraction of the oxidizing composition is injected at the secondinjection point 30, producing energy capable of increasing thetemperature to a value T1 ₂ according to the slope 46.

The same operation is repeated as many times as necessary, taking careto restrain the temperature of the reaction medium by the controlledinjection of the fractions of oxidizing composition.

Before the injection of the nth fraction of oxidizing agent into thetubular body 18 at the injection point 32, the temperature of thereaction medium must not be greater than the temperature T2, which islower than 800° K. This is because, in the contrary case, the risks ofdamage to the internal wall of the tubular body 18 are great, since thenth and final injection further increases the temperature of thereaction medium according to a slope 48.

The final injection of oxidizing composition makes possible thedecomposition of the organic substances of the aqueous effluents whichwere not decomposed during the preceding stages. In order to ensure amaximum yield of the oxidation reaction, the sum of the n fractions ofoxidizing composition is substantially greater than the stoichiometricamount necessary. Very clearly, as the process is continuous, it is thesum of the flow rates of the fraction of oxidizing composition withrespect to the flow rate of the aqueous effluent in the tubular body 18which corresponds to a greater than stoichiometric ratio.

Furthermore, as the heat capacity of water is at a maximum for atemperature substantially equal to 670° K., a large fraction ofoxidizing composition is advantageously injected within a temperaturerange for the reaction medium comprising this value of 670° K. This isbecause, since the heat capacity of water is at a maximum at this valueof the temperature, the thermal energy produced by the oxidationreaction is so much better absorbed, which restricts the increase in thetemperature of the reaction media and thus the damage to the internalwall of the tubular body 18.

In addition, when the oxidizing composition is oxygen, it is soluble inthe liquid phase of the aqueous effluent for all the injections. Thisadvantageous distinctive feature makes it possible to avoid hot spots inthe tubular body. This is because the complete solubility of the oxygenin the reaction medium makes possible a homogeneous and instantaneousdistribution of the oxidizing agent, which produces an increase intemperature throughout the reaction medium since the reactions beginsubstantially at the same time. Conversely, poor solubility of theoxidizing agent leads to localized reactions in the reaction media andtherefore to hot spots.

Reference will be made to FIGS. 3 and 4 in describing a specificembodiment comprising three injection points for three fractions ofoxidizing composition.

The plant in accordance with the invention and the thermal profile whichis associated with it are found in FIGS. 3 and 4. The aqueous effluentis injected under pressure through the inlet orifice 20. The preheatingmeans and the injection of the first fraction of oxidizing compositionat the injection point 24 allow the reaction medium to reach thetemperature T1 for a transient period corresponding to the plateau 50.The injection of the second fraction of oxidizing composition at theinjection point 30 produces an increase in the temperature to a value T2corresponding to the plateau 52. Subsequently, the final injection,which makes possible the oxidation of the organic substances which havenot yet reacted, raises the temperature of the reaction medium to atemperature substantially greater than T2. Very clearly, the values ofT1 and T2 are in this instance the same as the values T1 and T2mentioned in FIGS. 1 and 2.

According to another specific embodiment, which is not shown, whileretaining the principle described above according to which threefractions of oxidizing agent are injected, the injection of the firstfraction is carried out at an injection point situated in the tubularbody upstream of the preheating means close to the inlet orifice of thetubular body. Thus, the oxidizing composition constitutes, with theaqueous effluent comprising the organic substances, a reaction mediumwith a temperature substantially equal to the initial temperature of theaqueous effluent. The preheating means allow the oxidation reaction tobegin from the first rise in temperature of the reaction medium, whichis itself produced by the reaction.

According to a further embodiment, not shown, only two fractions ofoxidizing composition are injected. This configuration is advantageouswhen the concentration of organic substances in the aqueous effluent islow.

A specific example of the implementation of the invention is given byway of indication in the description which will follow.

The reactor or tubular body comprises four injection points and apreheater which allows the temperature of the aqueous effluent to bebrought to a temperature of 425° K.

The effluent to be treated is composed of a mixture of glucose andmethanol comprising 3.9% by weight of glucose and 4.9% of methanol in anaqueous phase. To completely oxidize this mixture, the amount of oxygennecessary is 88.9 g/l. This amount is known as the “chemical oxygendemand” or more usually COD. The amount injected in this instancecorresponds to a stoichiometry of 1.1.

The flow rate of the effluent in the reactor is 1 kg/hour at a pressureof 25 MPa.

The table represented below comprises the measurement of the length ofthe reactor in meters, the point 0 being substantially the point ofinjection of the aqueous effluent, the position of the injections of theoxygen fractions and the corresponding temperature of the reactor.

Oxygen flow rates Position in in g/l meters Temperature in ° K. 0 298 3425 18 4 423 5 477 25 6 525 7 570 25 8 633 9 650 25 10 679 11 695 12 82513 854 14 849 16 792

The example above is in no way limiting and it would not be departingfrom the scope of the invention to treat any other effluent compositionwith a different oxidizing agent and by means of a plant comprising adifferent number of injection points.

According to another aspect, the oxidation plant comprises means, notshown, for recovering the salts present in the aqueous effluents.

Thus, the tubular body is extended at its outlet by a second tubularbody into which the aqueous effluent and the salts present therein flowat a temperature of between 750 and 900° K., for example 820° K. Thesecond tubular body comprises an inlet nozzle into which water can beinjected to cool the aqueous effluent to a temperature of between 700and 800° K., for example 750° K.

The second tubular body emerges in a hopper-forming receptacle through apressure-reducing nozzle. The internal pressure of the receptacle beingbetween atmospheric pressure and said pressure P1, for example 1 MPa. Inthat way, the aqueous effluent comprising the salts is reduced inpressure, all the salts are converted to a solid state and the aqueouseffluent is converted to the vapor state. The salts can thus berecovered at the lower end of the hopper and the vapor at another outletinserted for this purpose at a temperature of between 500 and 600° K.,for example 550° K.

In addition, in a particularly advantageous way, the outlet of thetubular body and/or the second tubular body comprises an ultrasoundcleaning device, applied to the external walls, which makes it possibleto clean away the salts which sediment on the internal wall of thetubular bodies and which present a risk of blocking the tubular bodiesduring the oxidation process.

1. A process for the oxidation of organic substances present in anaqueous effluent, said aqueous effluent being capable of comprisingsalts, the process comprising the following stages: injecting saidaqueous effluent having an initial pressure and temperature, andcomprising a predetermined amount of organic substances, into a tubularbody having an inlet and an outlet; bringing said aqueous effluent to apressure P1 corresponding at least to the critical pressure of saidaqueous effluent, said pressure P1 being greater than the initialpressure; bringing said aqueous effluent to a temperature T1 greaterthan the initial temperature with heating means applied in a zone ofsaid tubular body; injecting into said tubular body at n points spacedapart from one another, n fractions of at least one oxidizingcomposition, whose sum corresponds to the amount of oxidizingcomposition necessary for the oxidation of said predetermined amount oforganic substances, so that a portion of the thermal energy produced bythe oxidation reaction increases the temperature of the reaction mixturefrom said temperature T1 to a temperature T2 which is greater than T1according to an increasing curve between said zone of said tubular bodyand the nth injection point, whereby said organic substances areoxidized, said reaction mixture continuously evolving from a subcriticalliquid state to the supercritical region; and wherein said process isimplemented without injecting a substance capable of simultaneouslycooling the reaction medium during the oxidation reaction.
 2. Theoxidation process according to claim 1, wherein said pressure P1 of saidaqueous effluent is greater than 23 MPa and the temperature T1 of saideffluent is between 370 and 520° K.
 3. The oxidation process accordingto claim 1, wherein the portion of thermal energy produced by theoxidation reaction increases the temperature of said reaction mixture toa temperature T2 of less than 800° K.
 4. The oxidation process accordingto claim 1, wherein three fractions of an oxidizing composition areinjected into said tubular body at three points spaced apart from oneanother.
 5. The oxidation process according to claim 1, wherein thefirst fraction of oxidizing composition is injected after said aqueouseffluent has reached the temperature T1.
 6. The oxidation processaccording to claim 1, further comprising imparting a portion of thethermal energy produced by said oxidation to said aqueous effluent inorder to bring said aqueous effluent to said temperature T1.
 7. Theoxidation process according to claim 1, wherein the oxidizingcomposition is oxygen.
 8. The oxidation process according to claim 1,wherein the oxidizing composition is hydrogen peroxide.
 9. The oxidationprocess according to claim 1, wherein at least one of the fractions ofoxidizing composition is composed of an oxidizing composition which isdifferent in nature from the other fractions.
 10. The oxidation processaccording to claim 1, further comprising the following stages:recovering said aqueous effluent and the salts present therein at theoutlet of the tubular body; lowering the pressure of said aqueouseffluent from said pressure P1 to a pressure P0, ranging betweenatmospheric pressure and said pressure P1, so as to convert all thesalts to the solid state and said aqueous effluent to the vapor state;recovering the salts in the solid state; and recovering said aqueouseffluent in the vapor state, whereby said aqueous effluent and the saltspresent therein are physically separated.